1. Field of the Invention
The present invention relates to a method of repairing or reworking radial mounting tabs on the rim of turbomachine disks. It is aimed in particular at the retaining lugs and, more particularly still, at the so-called central retaining lugs that hold inter-blade platforms on a fan disk in a multiple-flow turbojet engine.
2. Description of the Related Art
A known double-flow or turbofan engine, like those of the CFM 56 family, comprises a front fan driven by the low-pressure turbine of the associated gas generator. The fan disk comprises a rim around the periphery and in which substantially axially oriented cavities are machined and is secured downstream to a low-pressure compressor drum for the primary flow fed into the gas generator. The disk and the drum form the low-pressure compressor module. The cavities are of dovetail cross section and act as housings for the fan blade roots. The rim also supports inter-blade platforms which delimit the radially inner wall of the duct for the flow of air passing through the fan. The terms axial and radial, inner and outer, upstream and downstream are defined, in this text, with respect to the axis of the engine on which the various denoted components are mounted and with respect to the direction of the gaseous flow passing through this engine.
Each platform comprises a plate of elongate shape which presses laterally against the adjacent fan blades with, on its inner face, means of attachment to the rim and to the drum which are formed of inner radial tabs each pierced with an axial bore. The rim and the drum comprise means of attachment of the platforms, these means being formed of radial tabs through which there pass axial pins and a head of which projects with respect to the tab through which it has passed. More specifically, the platforms are centered and retained by the fixing means at three fixing points on the low-pressure compressor module. Two of these fixing points are situated on the fan disk and the third, which is further downstream, can be found on the low-pressure drum. The inner radial tabs of the platforms are slipped onto the heads of the pins which are secured to the rotor.
One example of a fan rotor arrangement with inter-blade platforms retained by inner radial tabs on a fan disk, at three points of attachment one of which is arranged centrally between an upstream retaining means and a downstream retaining means is described in U.S. Pat. No. 6,447,250.
When the engine is running, the platform retaining zones are subjected to high radial loads.
The present Applicant has, in Patent EP 1 503 039, described an inter-blade platform for a turbojet engine fan disk equipped with at least one first fixing tab provided with an orifice through which a fixing pin intended to connect it to a second fixing tab of the disk can pass; the fixing pin comprises a shank with a threaded portion having a first diameter and a second part comprising a flange extending the first part, having a second diameter greater than the first diameter, and intended to be interposed between the first and second fixing tabs, and a head extending the first sub-part and having a third diameter smaller than the second diameter so that it can pass through the orifice in the first fixing tab. An arrangement such as this allows a better distribution of load between the pins and the fixing tabs. However, it involves altering the geometry of the components currently used and cannot be used as a substitute for the components. Further, that document neither discloses nor suggests any repair method.
In one type of engine, the pins comprise a head and a shank with a threaded portion. The head has a shoulder pressing against the upstream face of the outer radial tab of the rim. The force with which the head presses against the surface comes as a result of the tightening of a nut which is screwed onto the shank with a threaded portion against the opposite face. It may be found that the central zone of attachment between the outer radial tab for attachment to the rim, positioned between the upstream and downstream fixing means, and the radially internal central fixing tab of the platform is susceptible to wear and to the onset of cracking. Such damage can be explained by the tiny movements of the retaining pin during operation. This pin is slipped into the bore of the outer radial tab of the rim and as mentioned hereinabove is held therein by screw-fastening. However, the force with which the pin is tightened is limited by the mechanical strength of its working cross section. The centrifugal forces experienced during operation may exceed the level of tightening and cause such movements. These movements cause wearing of the tab at the site where the head of the pin is in contact with and presses against it. In addition, cracking of the tab at the entrance to the bore accommodating the retaining pin has been noticed.